The inter-frame encoding characterized in that frames of a video image signal are closely interrelated is widely used as a technique for data compression in video image signals. So far were introduced various methods of the inter-frame encoding, many of which are featured by encoding an inter-frame difference of a video image signal. In an encoding technique often employed as the inter-frame encoding, orthogonal transform is used, in which two-dimensional image correlation is efficiently availed in a differential signal. The internationally standardized video image encoding for storage media such as ISO/IEC 11172-2 (generally called MPEG1), in particular, employs an encoding technique based on DCT (Discrete Cosine Transform) which is an example of the orthogonal transform.
In the case where a target frame is an inter-frame encoding frame, the inter-frame encoding technique calculates an inter-frame differential value of an image signal and transforms the calculated differential value into a variable code based on, for example, Huffman encoding. The inter-frame encoding in which the differential value thus transformed increases inter-frame correlation after motion compensation because of the characteristics of a video image, therefore, allocates a short variable code to 0 or a differential value approximate to 0 so that a compression efficiency is improved. It is unnecessary to encode any data of divided blocks resulting in 0 after motion compensating prediction, which further improves the compression efficiency.
It is an indispensable step in data compression of video image signals to adjust an encoding amount (rate control). A known technique to adjust the encoding amount is to change a quantization step, more specifically, the quantization step used in data encoding is decided by monitoring a generated encoding amount. A virtual buffer which accepts generated codes and outputs them based on a constant rate (fixed rate) is prepared, and the quantization step is calculated in proportion to a level of buffer fullness. More specifically, the quantization step is increased when the buffer fullness is elevated, while the quantization step is decreased when the buffer fullness goes down, so that the encoding is decelerated with a large quantization step and the encoding is accelerated with a small quantization step. Therefore, the generated encoding amount can be constantly regulated on average when the buffer fullness is controlled to stay at, for example, around 50%.
In any scene where there is a large motion, the differential encoding becomes difficult, increasing the encoding amount even if the quantization step is changed. Although the quantization step is increased to adjust the encoding amount, an image quality is unavoidably deteriorated. To avoid the image quality deterioration when the quantization step is changed to adjust the encoding amount, for example, the motion of each frame is evaluated based on information on camera shake or motion vectors, and the quantization step is increased in any frame decided as having a large motion as compared to a preceding frame so that the encoding amount is controlled.
In the quantization step change method, a fixed rate is used to control the encoding amount. Apart from controlling the encoding amount based on a fixed rate, a variable rate may be employed, in which the quantization step is fixed to allow the encoding amount to be variable. More specifically, the quantization step is decided in advance so that the image quality does not fall below an expected quality level and then fixed for the encoding process.